"[A]fter controlling for education and other characteristics, there were no significant differences associated with cannabis consumption"

Melbourne, Australia: The consumption of cannabis, even long-term, poses few adverse effects on cognitive performance, according to clinical trial data to be published in the scientific journal Addiction.

Investigators at the University of Melbourne and the Australian National University, Center for Mental Health Research assessed the impact of cannabis use on various measures of memory and intelligence in over 2,000 self-identified marijuana consumers and non-users over an eight-year period. Among cannabis consumers, subjects were grouped into the following categories: 'heavy' (once a week or more) users, 'light' users, 'former heavy' users, 'former light' users, and 'always former' – a category that consisted of respondents who had ceased using marijuana prior to their entry into the study.

Researchers reported: "Only with respect to the immediate recall measure was there evidence of an improved performance associated with sustained abstinence from cannabis, with outcomes similar to those who had never used cannabis at the end point. On the remaining cognitive measures, after controlling for education and other characteristics, there were no significant differences associated with cannabis consumption."

They concluded, "Therefore, the adverse impacts of cannabis use on cognitive functions either appear to be related to pre-existing factors or are reversible in this community cohort even after potentially extended periods of use."

Separate studies have previously reported that long-term marijuana use is not associated with residual deficits in neurocognitive function. Specifically, a 2001 study published in the journal Archives of General Psychiatry found that chronic cannabis consumers who abstained from the drug for one week "showed virtually no significant differences from control subjects (those who had smoked marijuana less than 50 times in their lives) on a battery of 10 neuropsychological tests. ... Former heavy users, who had consumed little or no cannabis in the three months before testing, [also] showed no significant differences from control subjects on any of these tests on any of the testing days."

Additionally, studies have also implied that cannabis may be neuroprotective against alcohol-induced cognitive deficits. A 2009 study by investigators at the University of California at San Diego reported that binge drinkers who also used cannabis experienced significantly less white matter damage to the brain as compared to subjects who consumed alcohol alone.

For more information, please contact Paul Armentano, NORML Deputy Director, at: paul@norml.org. Full text of the study, "Cannabis use and cognitive function: eight year trajectory in a young adult cohort," is available online from the journal Addiction.